Making a particle effect parallel to the slope of a terrain

I want to instantiate a particle effect and make it parallel to a slope, but I do not know how to do it.

I don’t get quite what you mean by the question please can you be a bit more specific. If your meaning that you have a constant gradient slope and you want particles to follow the slope as shown below.

First you need to find the incline of the slope. do Mathf.Atan(heightChange / widthChange) * Mathf.RadToDeg.

Then take that angle and instantiate the particle system at that angle.
Quaternion InstantiateAngle = Quaternion.Euler(new Vector3(0f,0f, angle));
Instantiate(particleSystemPrefab, transform.position, InstanceAngle);

Btw the code has not been tested so there may be a few mistakes but the principle should work.